1. Field of the Invention. The invention relates in general to adaptive filtering techniques and, in particular, to an analog adaptive filter using an acoustic surface wave (ASW) storage correlator.
2. Description of Prior Art. Adaptive filtering (J. M. McCool and B. Widrow, "Principles and Applications of Adaptive Filters," IEE Conf. Publ., 144, pp. 84-95, 1976) is useful in removing distortion from signals, particularly when the distortion varies in time. Adaptive filters have been used to perform deconvolution of a distorted echo pulse in an acoustic imaging system (D. Corl, "A.C.T.D. Adaptive Inverse Filter," Electr. Lett., 14. pp. 60-62, 1978), to equalize the distortion in a telephone channel (R. W. Lucky, "Automatic Equalization for Digital Communications," Bell Sys. Tech. J., 66, pp. 547-588, 1965), and to suppress an interfering signal (P. M. Grant and G. S. Kino, "Adaptive Filter Based on SAW Monolithic Storage Correlators," Electr. Lett., 14, pp/ 562-564, 1978).
Most adaptive filters have been implemented using digital techniques. The limitations of the digital approach are the limited bandwidth (typically 10 MHz) and the practical limit on the number of taps dictated by the complexity and power consumption of systems with large numbers.
An analog-digital hybrid approach has been implemented using MOS LSI technology (B. K. Ahuja, et al., "A Sampled Analog MOS LSI Adaptive Filter," IEEE J. Solid-State Cir., Sc-14, pp. 148-154, 1979). This has the advantage of lowering the power consumption and allowing 32 taps to be used without undue external complexity. Large dynamic range was obtained with this technique (60 dB), but the bandwidth was limited to less than 1 MHz.
Most analog implementations of an adaptive filter have been made using charge coupled devices with analog tap weights held in sample and hold circuits (D. F. Barbe, et al., "Signal Processing with Charge Coupled Devices," IEEE J. Solid-State Cir., Sc-13, pp. 34-51, 1978). The limitations here are the narrow bandwidth and variation across the chip in the gain and threshold levels. An alternative approach for implementing a wideband adaptive filter is to employ tapped ASW filters with complex computer-controlled systems for adjusting the tap weights (W. K. Masenten, "Adaptive Processing for Spread Spectrum Communications Systems," Report TP 77-14-22. Hughes Aircraft Co., September 1977).